A decrease in gas turbine load during operation of the gas turbine can cause a rapid increase in the rotational speed of the gas turbine.
For example, in a plant including a power generating facility which is grid-connected with a commercial power system and drives a generator by a gas turbine, and a load facility which consumes power, the rotational speed of the gas turbine increases when the power generating facility is isolated from the commercial power system and the power generating facility shifts to isolated operation of transmitting power only to the load facility in the plant. In this case, the demand for gas turbine output rapidly decreases (load decrease) by an amount corresponding to the loss of load under which the power generating facility would otherwise transmit power to the commercial power system. This makes it necessary to rapidly throttle the fuel to be supplied to a combustor of the gas turbine.
In actual operation, however, reduction in the mechanical output of the gas turbine can be delayed due to the following reasons: a delay in detection of the load decrease; supply of the fuel remaining in a fuel passage to the combustor; a delay in action of a flow rate regulating valve for regulating the fuel flow rate or a pressure regulating valve for regulating the fuel pressure; or other causes, which can result in an increase in frequency of the power system in the plant.
Therefore, when a shift to isolated operation occurs, it is necessary to suppress an increase in frequency as well as to secure stability of combustion.
In view of this, PTL 1 describes a gas turbine fuel control device, which, upon occurrence of isolation from a load or isolation from a power transmission system, sets a fuel flow rate in a predetermined premixed combustion fuel system of multiple premixed combustion fuel systems to a predetermined first minimum fuel flow rate required for flame retention, for a first effective time, and sets a fuel flow rate in a diffusion combustion fuel system to a predetermined second minimum fuel flow rate required for flame retention, for a second effective time.